1 //===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 /// 10 /// \file 11 /// This file contains the WebAssembly implementation of 12 /// TargetFrameLowering class. 13 /// 14 /// On WebAssembly, there aren't a lot of things to do here. There are no 15 /// callee-saved registers to save, and no spill slots. 16 /// 17 /// The stack grows downward. 18 /// 19 //===----------------------------------------------------------------------===// 20 21 #include "WebAssemblyFrameLowering.h" 22 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h" 23 #include "WebAssemblyInstrInfo.h" 24 #include "WebAssemblyMachineFunctionInfo.h" 25 #include "WebAssemblySubtarget.h" 26 #include "WebAssemblyTargetMachine.h" 27 #include "WebAssemblyUtilities.h" 28 #include "llvm/CodeGen/MachineFrameInfo.h" 29 #include "llvm/CodeGen/MachineFunction.h" 30 #include "llvm/CodeGen/MachineInstrBuilder.h" 31 #include "llvm/CodeGen/MachineModuleInfoImpls.h" 32 #include "llvm/CodeGen/MachineRegisterInfo.h" 33 #include "llvm/MC/MCAsmInfo.h" 34 #include "llvm/Support/Debug.h" 35 using namespace llvm; 36 37 #define DEBUG_TYPE "wasm-frame-info" 38 39 // TODO: wasm64 40 // TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions 41 42 /// We need a base pointer in the case of having items on the stack that 43 /// require stricter alignment than the stack pointer itself. Because we need 44 /// to shift the stack pointer by some unknown amount to force the alignment, 45 /// we need to record the value of the stack pointer on entry to the function. 46 bool WebAssemblyFrameLowering::hasBP( 47 const MachineFunction &MF) const { 48 const auto *RegInfo = 49 MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo(); 50 return RegInfo->needsStackRealignment(MF); 51 } 52 53 /// Return true if the specified function should have a dedicated frame pointer 54 /// register. 55 bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const { 56 const MachineFrameInfo &MFI = MF.getFrameInfo(); 57 58 // When we have var-sized objects, we move the stack pointer by an unknown 59 // amount, and need to emit a frame pointer to restore the stack to where we 60 // were on function entry. 61 // If we already need a base pointer, we use that to fix up the stack pointer. 62 // If there are no fixed-size objects, we would have no use of a frame 63 // pointer, and thus should not emit one. 64 bool HasFixedSizedObjects = MFI.getStackSize() > 0; 65 bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects; 66 67 return MFI.isFrameAddressTaken() || 68 (MFI.hasVarSizedObjects() && NeedsFixedReference) || 69 MFI.hasStackMap() || MFI.hasPatchPoint(); 70 } 71 72 /// Under normal circumstances, when a frame pointer is not required, we reserve 73 /// argument space for call sites in the function immediately on entry to the 74 /// current function. This eliminates the need for add/sub sp brackets around 75 /// call sites. Returns true if the call frame is included as part of the stack 76 /// frame. 77 bool WebAssemblyFrameLowering::hasReservedCallFrame( 78 const MachineFunction &MF) const { 79 return !MF.getFrameInfo().hasVarSizedObjects(); 80 } 81 82 // Returns true if this function needs a local user-space stack pointer for its 83 // local frame (not for exception handling). 84 bool WebAssemblyFrameLowering::needsSPForLocalFrame( 85 const MachineFunction &MF) const { 86 auto &MFI = MF.getFrameInfo(); 87 return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF); 88 } 89 90 // In function with EH pads, we need to make a copy of the value of 91 // __stack_pointer global in SP32 register, in order to use it when restoring 92 // __stack_pointer after an exception is caught. 93 bool WebAssemblyFrameLowering::needsPrologForEH( 94 const MachineFunction &MF) const { 95 auto EHType = MF.getTarget().getMCAsmInfo()->getExceptionHandlingType(); 96 return EHType == ExceptionHandling::Wasm && 97 MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls(); 98 } 99 100 /// Returns true if this function needs a local user-space stack pointer. 101 /// Unlike a machine stack pointer, the wasm user stack pointer is a global 102 /// variable, so it is loaded into a register in the prolog. 103 bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF) const { 104 return needsSPForLocalFrame(MF) || needsPrologForEH(MF); 105 } 106 107 /// Returns true if the local user-space stack pointer needs to be written back 108 /// to __stack_pointer global by this function (this is not meaningful if 109 /// needsSP is false). If false, the stack red zone can be used and only a local 110 /// SP is needed. 111 bool WebAssemblyFrameLowering::needsSPWriteback( 112 const MachineFunction &MF) const { 113 auto &MFI = MF.getFrameInfo(); 114 assert(needsSP(MF)); 115 // When we don't need a local stack pointer for its local frame but only to 116 // support EH, we don't need to write SP back in the epilog, because we don't 117 // bump down the stack pointer in the prolog. We need to write SP back in the 118 // epilog only if 119 // 1. We need SP not only for EH support but also because we actually use 120 // stack or we have a frame address taken. 121 // 2. We cannot use the red zone. 122 bool CanUseRedZone = MFI.getStackSize() <= RedZoneSize && !MFI.hasCalls() && 123 !MF.getFunction().hasFnAttribute(Attribute::NoRedZone); 124 return needsSPForLocalFrame(MF) && !CanUseRedZone; 125 } 126 127 void WebAssemblyFrameLowering::writeSPToGlobal( 128 unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB, 129 MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const { 130 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 131 132 const char *ES = "__stack_pointer"; 133 auto *SPSymbol = MF.createExternalSymbolName(ES); 134 BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::SET_GLOBAL_I32)) 135 .addExternalSymbol(SPSymbol, WebAssemblyII::MO_SYMBOL_GLOBAL) 136 .addReg(SrcReg); 137 } 138 139 MachineBasicBlock::iterator 140 WebAssemblyFrameLowering::eliminateCallFramePseudoInstr( 141 MachineFunction &MF, MachineBasicBlock &MBB, 142 MachineBasicBlock::iterator I) const { 143 assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) && 144 "Call frame pseudos should only be used for dynamic stack adjustment"); 145 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 146 if (I->getOpcode() == TII->getCallFrameDestroyOpcode() && 147 needsSPWriteback(MF)) { 148 DebugLoc DL = I->getDebugLoc(); 149 writeSPToGlobal(WebAssembly::SP32, MF, MBB, I, DL); 150 } 151 return MBB.erase(I); 152 } 153 154 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF, 155 MachineBasicBlock &MBB) const { 156 // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions 157 auto &MFI = MF.getFrameInfo(); 158 assert(MFI.getCalleeSavedInfo().empty() && 159 "WebAssembly should not have callee-saved registers"); 160 161 if (!needsSP(MF)) return; 162 uint64_t StackSize = MFI.getStackSize(); 163 164 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 165 auto &MRI = MF.getRegInfo(); 166 167 auto InsertPt = MBB.begin(); 168 while (InsertPt != MBB.end() && WebAssembly::isArgument(*InsertPt)) 169 ++InsertPt; 170 DebugLoc DL; 171 172 const TargetRegisterClass *PtrRC = 173 MRI.getTargetRegisterInfo()->getPointerRegClass(MF); 174 unsigned SPReg = WebAssembly::SP32; 175 if (StackSize) 176 SPReg = MRI.createVirtualRegister(PtrRC); 177 178 const char *ES = "__stack_pointer"; 179 auto *SPSymbol = MF.createExternalSymbolName(ES); 180 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::GET_GLOBAL_I32), SPReg) 181 .addExternalSymbol(SPSymbol, WebAssemblyII::MO_SYMBOL_GLOBAL); 182 183 bool HasBP = hasBP(MF); 184 if (HasBP) { 185 auto FI = MF.getInfo<WebAssemblyFunctionInfo>(); 186 unsigned BasePtr = MRI.createVirtualRegister(PtrRC); 187 FI->setBasePointerVreg(BasePtr); 188 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr) 189 .addReg(SPReg); 190 } 191 if (StackSize) { 192 // Subtract the frame size 193 unsigned OffsetReg = MRI.createVirtualRegister(PtrRC); 194 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 195 .addImm(StackSize); 196 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::SUB_I32), 197 WebAssembly::SP32) 198 .addReg(SPReg) 199 .addReg(OffsetReg); 200 } 201 if (HasBP) { 202 unsigned BitmaskReg = MRI.createVirtualRegister(PtrRC); 203 unsigned Alignment = MFI.getMaxAlignment(); 204 assert((1u << countTrailingZeros(Alignment)) == Alignment && 205 "Alignment must be a power of 2"); 206 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), BitmaskReg) 207 .addImm((int)~(Alignment - 1)); 208 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::AND_I32), 209 WebAssembly::SP32) 210 .addReg(WebAssembly::SP32) 211 .addReg(BitmaskReg); 212 } 213 if (hasFP(MF)) { 214 // Unlike most conventional targets (where FP points to the saved FP), 215 // FP points to the bottom of the fixed-size locals, so we can use positive 216 // offsets in load/store instructions. 217 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), 218 WebAssembly::FP32) 219 .addReg(WebAssembly::SP32); 220 } 221 if (StackSize && needsSPWriteback(MF)) { 222 writeSPToGlobal(WebAssembly::SP32, MF, MBB, InsertPt, DL); 223 } 224 } 225 226 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF, 227 MachineBasicBlock &MBB) const { 228 uint64_t StackSize = MF.getFrameInfo().getStackSize(); 229 if (!needsSP(MF) || !needsSPWriteback(MF)) return; 230 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 231 auto &MRI = MF.getRegInfo(); 232 auto InsertPt = MBB.getFirstTerminator(); 233 DebugLoc DL; 234 235 if (InsertPt != MBB.end()) 236 DL = InsertPt->getDebugLoc(); 237 238 // Restore the stack pointer. If we had fixed-size locals, add the offset 239 // subtracted in the prolog. 240 unsigned SPReg = 0; 241 if (hasBP(MF)) { 242 auto FI = MF.getInfo<WebAssemblyFunctionInfo>(); 243 SPReg = FI->getBasePointerVreg(); 244 } else if (StackSize) { 245 const TargetRegisterClass *PtrRC = 246 MRI.getTargetRegisterInfo()->getPointerRegClass(MF); 247 unsigned OffsetReg = MRI.createVirtualRegister(PtrRC); 248 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 249 .addImm(StackSize); 250 // In the epilog we don't need to write the result back to the SP32 physreg 251 // because it won't be used again. We can use a stackified register instead. 252 SPReg = MRI.createVirtualRegister(PtrRC); 253 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), SPReg) 254 .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32) 255 .addReg(OffsetReg); 256 } else { 257 SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32; 258 } 259 260 writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL); 261 } 262